This invention relates to electromagnetic energy collection and more particularly to devices and systems for the collection and recovery of solar energy or the like. Solar energy is an increasingly important source of energy for space heating and cooling, process heat, electrical generation and the like. Relatively low temperatures are useful for space heating, but cooling and process applications generally require higher temperatures. Direct photovoltaic conversion to electricity is known but existing photovoltaic materials such as silicone wafer materials are currently expensive. Since solar insolation per unit of area is low, high efficiency and concentration are desirable.
Most of the solar energy collectors heretofore employed have been flat plate collectors without concentration. While simple, such systems are limited in efficiency and in the temperatures obtainable. Focusing collectors, that is those which concentrate solar energy by forming an image of the sun, are known and are capable of very high temperatures. However they can utilize only direct beam rays and require continuous tracking of the daily and seasonal solar movement. They cannot collect diffuse, scattered sunlight.
Improved nonfocusing concentrating collectors have been recently disclosed in U.S. Pat. No. 3,923,381 and further described in an article entitled: "Solar Energy Research and Development" published by the Argonne National Laboratory, dated June 19, 1976. Those references contain a good description of the different types of collectors and the advantages and applications for each. However, the nonfocusing collectors described have decreasing acceptance angles with increasing concentration and require means for repositioning monthly or seasonally for concentration ratios greater than about 2.
There is a continuing need in the solar energy field for a concentrating collector system which provides a concentration ratio of 3 to 6 or more and which is efficient at a sufficiently wide angle of incidence to permit use as a totally stationary collector. Moreover, with concentrating collectors capable of achieving substantially elevated temperatures, means are needed to prevent the buildup of dangerously elevated temperatures and pressures when the heat is not needed or the cooling system is inoperative.
Accordingly, principal objects of the present invention include provision of nonfocusing radiant energy collector systems and devices which are efficient at a sufficiently wide incident angle to be useful at concentration ratios of three or more as a stationary system, which minimize loss of absorbed energy, and which provide a simple and convenient means for controlling temperature.